1. Field of the Invention
This invention relates to apparatus and process for fluidized solids systems for use in chemical and thermal processes. More particularly, this invention relates to attaining homogeneous, dense fluidized bed conditions at high superficial gas velocities with controllable back mixing of gases and solids by use of multiple vertically arranged fluidized beds interconnected by downcomer pipes extending from the lower portion of an upper bed to the lower portion of a lower bed. High amounts of solids circulation between vertically disposed fluidized beds aid in homogeneous fluidization at higher superficial gas velocities.
2. Description of the Prior Art
Fluidized beds of solids are used in a wide variety of processes for improved fluid-solid contacting to enhance physical reactions, such as heat transfer, and chemical reactions between gases and solids and to provide a media for conduct of chemical reactions utilizing solid catalysts.
A major difficulty with prior fluidized bed reactors has been scaling them up from laboratory units to industrial plants. The most important requirement for successful scale-up is to retain a homogeneous fluidized bed since in a non-homogeneous fluidized bed the size of the bed affects the behavior of the solids as more fully described in Van Heerden, J. J., "Mixing Patterns in Large-Scale Fluidized Bed", in Grace, J. R. and Matsen, J. F., Eds., Fluidization, 69. New York: Plenum Press (1980). A homogeneous fluidized bed has been defined as one in which the bubbles are relatively small and both the circulation flow of solids and the gas-solid contact are intense and in which the minimum bubble formation velocity is higher than the minimum fluidization velocity. For further explanation of the types of gas fluidization see Geldart, D., "Type of Gas Fluidization", Powder Technology 1, 285 (1972). Solid particles having a mean diameter of less than about 100 micrometers must be employed to obtain a homogeneous fluidization state as pointed out more fully in Van Swaaij, W. P. M., "Gas-Solids Fluid Bed Reactors", A.C.S. Symp. Ser. 72, 1973 (1978). However, it has been found that even with utilization of such fine particles the undesired formation of large bubbles cannot be avoided in the fluidized bed as more fully pointed out in Yerushalmi, J. and Squires, A. M., "The Phenomenon of Fast Fluidization", A.I.Ch.E. Symp. Ser. 73, 44 (1977). In an attempt to solve this problem, Yerushalmi and Squires proposed a very fast fluidized bed which, however, has the disadvantages that there is no dense phase and that a large amount of the solid particles are carried from the fluidized bed and must be recycled through a separator, such as a cyclone. Use of internal baffles for controlling gas flow pattern in fluid bed reactors has been summarized by Harrison, D. and Grace; J. R., "Fluidized Bed with Internal Baffles", in Davidson, J. F. and Harrison D. Fluidization. 559, New York: Academic Press, (1971). A disadvantage of using conventional internal baffles is the inherent sacrifice of a homogeneous fluidization state. On the other hand, it has been proposed to use baffled multi-stage fluidized beds using coarse solid particles to attain a countercurrent contact of gas and solids in the two stages as described in Guigon, P., et al, "Particle Interchange Between Stages in a Baffled Fluidized Bed", A.I.Ch.E. Symp. Ser. 70, 63 (1974). The high solids interchange rate between two stages of a baffle fluidized bed is not conducive to homogeneous fluidization but is an attempt to achieve countercurrent flow of solids and gas.
Multiple fluidized beds arranged vertically have been utilized with solids transfer between beds. The prior art has also taught various systems of baffles for the mechanical distribution of fluids to obtain better fluid-solids contact. It is known to the art to have multiple vertically arranged fluidized beds with overflow downcomers with solids from an upper bed overflowing into the downcomer for transport to a lower fluidized bed. U.S. Pat. No. 2,639,973 teaches overflow downcomers providing solids to the upper portion of a lower bed; U.S. Pat. Nos. 2,684,840; 2,698,321; 2,891,846; 2,890,106; 3,910,769 and French Patent No. 1,058,923 teach overflow downcomers for transfer of solids from the upper surface of an upper fluidized bed to the central or lower portion of a lower fluidized bed. U.S. Pat. No. 4,017,585 teaches transfer of solids from the lower portion of an upper fluidized bed to above the top surface of a lower fluidized bed. The cited patents recognize the desirability for improving the gas-solids contacting characteristics of fluidized beds and desirability of control of the fluidized bed reaction conditions.
Solids transfer for heat supply between two vertically arranged fluidized beds utilizing riser pipes from the upper portion of a lower fluidized bed to the lower portion of an adjacent upper fluidized bed and downcomers from the lower portion of an upper fluidized bed to the lower portion of an adjacent lower fluidized bed have been suggested in the publication Kono, H. and Ninomiya, K., "Heat Supply System for a Fluidized Bed Reactor Featuring an Endothermic Reaction", AIChE 20th National Heat Transfer Conference, Milwaukee, Wis., Aug. 2-5, 1981.